Chemical sensors, used for detection of variety of chemicals, such as chemical warfare agents, explosive agents and/or pollutants, especially when present as vapors, typically utilize spectrometry systems (such as mass spectrometry (MS)), and/or gas chromatography (GC), as well as electrochemical systems such as surface acoustic wave sensors (SAWS), mass sensors and optical sensors. The need for multiple sensor technologies is driven by the fact that there are a variety of chemical compounds/agents of interest that may need to be detected simultaneously and a variety of desired monitoring conditions and environments in which such chemicals may be found. This issue is further complicated by findings of a recent DARPA study of chemical and biological sensor standards that indicates that the minimal detectable concentration range of interest for chemical/biological agents of primary concern vary by more than 106.
Many of the chemical agents of interest, such as explosive agents, have a very low vapor pressure, for example, as low as 0.1-1.0 millitorr, which makes it difficult to detect such agents, except when present in large concentrations. Thus, sensitivity is a key consideration in chemical sensor development. Regardless of the sensor type, pre-concentrators may be used to control the input airflow to a detector so that the concentration of an agent/compound of interest is optimized for the detection sensitivity.
Typical pre-concentrators for vapor sensing systems rely on collection of the gas in a stainless steel canister or adsorption of the compound/agent (analyte) to be analyzed on a filter. The analyte is typically concentrated using cryogenic techniques (cooling) or pressurization (in the case of the canister method), or with the use of heat or a solvent (in the case of the filter method). These methods rely on long collection time and significant external infrastructure to activate and/or release the compound of interest (analyte).
Furthermore, complicated contact requirements between a pre-concentrator and the external activation and/or release mechanisms are generally required. For example, pre-concentrator units containing electrospun fiber sorbents typically depend on contact with an external heater for heating. These pre-concentrators are limited by uneven heating (due to the contact requirements) which causes gradual or partial release of analytes of interest that may be bound to the pre-concentrator, resulting in inefficient design, sensing and detecting abilities.
On the other hand, use of electrospun fibers for chemical detection has been limited to use of the fiber itself as the detection mechanism. For example, changes to an electrospun fiber's optical fluorescence, resistivity or impedance have been monitored to determine whether an analyte has bound to an electrospun fiber. While such systems may be sensitive and may have a fast response time, they are prone to high rates of false positives and are hampered by limited specificity of detection. Changes in fiber properties are a result of binding rather than binding by a specific analyte. Thus, binding of agents that may not be analytes of interest may also be detected by electrospun fiber based chemical detection systems.